As described in said application Ser. No. 306,175 those familiar with the operation and maintenance of steam generators understand that, during system downtime, the tubes of the steam generator must be inspected, and defective tubes plugged or repaired. It is also known that steam generator tubes may be preventively plugged, in advance of leaks actually occurring therein, if an examination of the interior of the tube indicates the presence of high strain or tube wall defect which, in turn, indicates possible incipient breakage. It is therefore an object of the present invention to provide a more effective means of examining tube interiors to detect the presence or not of such strain or defect which, in turn, will indicate the likelihood or not for tube breakage during reactivated use of the tube.
Distortion of the tube interior shape, commonly called "denting", is known to be a result of non-uniform strain in the metal of which the tube is made. High strain makes the tube susceptible to stress corrosion cracking which in its early state is impossible to detect by usual eddy current examination means because the "dented" distortion of the tube produces an over-riding eddy current signal. Determination of such cracking or other defect by more accurate eddy current examination of a distorted tube by which predictions regarding tube cracking can be improved is an important object of this invention. Thus, it is intended by the present invention to provide an interior probe for tubes, such as steam generator tubes, for more accurately sensing tube wall defects in distorted or "dented" tubes.
For example, especially in modern pressurized water reactor steam generators, "denting" occurs during operation by reason of an accumulation of corrosion products between the tube support plates and the tubes themselves. This distortion results in strain which, if high enough, makes the tube susceptible to stress corrosion cracking. In the past, eddy current inspection techniques have been used to evaluate the size of these "dents". Such non-destructive examination of steam generator tubes has been developed from previously known eddy current techniques for detecting defects in the tubing, such as existing cracks and the like, and the interpretation of eddy current signals indicating the existence of denting is based on a comparison with known signals from standards. Although denting can thus be quantified, eddy current measurements for this purpose have been found to be generally incorrect because, at best, they measure only the average tube diameter at any given location within the length of the tube. The eddy current signal resulting from even a small dent is large in comparison to the signal resulting from a small crack, so that the latter signal is not apparent in the presence of the former. Accordingly, tube leaks have not been accurately predicted using eddy current measurements.